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ENGINEERING  NEWS  REPRINTS 


25  Cents 


IGNITIONS  AND 
EXPLOSIONS  IN  THE 
DISCHARGE  PIPES 
AND  RECEIVERS  OF 
AIR  COMPRESSORS 


GOW 


Reprinted  from  ENGINEERING  NBWS,  March  2,  1905 


m 


Ignitions  and  Explosions  in 

the  Discharge  Pipes  and 

Receivers  of  Air 

Compressors 


BY 


ALEXANDER  M.  GOW,  M.  E. 

Assistant  Mechanical  Engineer,   Oliver    Mining   Co., 
Duluth,  Minn. 


(Reprinted  from  Engineering  News,  March  a,  1905) 


PRICE  25  CENTS 


NEW  YORK 
THE  ENGINEERING  NEWS  PUBLISHING  CO. 


GENERAL 


Copyright,  1905,  by  Engineering  News  Pub.  Co. 


IGNITIONS  AND  EXPLOSIONS  IN  THE  DISCHARGE  PIPES 
AND  RECEIVERS  OF  AIR  COMPRESSORS. 

Ignitions  and  explosions  in  the  discharge  pipes 
and  receivers  of  air  compressors  are  by  no  means 
uncommon.  The  increasing  use  of  compressed  air 
at  higher  and  higher  pressures,  together  with  the 
general  ignorance  on  the  part  of  the  men  in 
charge  of  the  machines,  as  to  how  and  why  such 
accidents  occur,  make  it  probable  that  they  will 
continue  to  occur  with  greater  frequency.  For 
very  obvious  reasons  the  manufacturers  say  as 
little  as  possible  about  the  subject  to  prospective 
buyers.  It  cannot  be  expected  that  the  men  in 
charge  at  the  time  of  accident  will  make  any  re- 
port that  will  reflect  upon  themselves.  So  it  • 
happens  that  in  but  comparatively  few  cases  of 
these  accidents  have  the  results  of  careful  investi- 
gation been  given  to  the  public  at  large. 

(From  an  examination  of  about  twenty  acci- 
dents, of  which  more  or  less  complete  data  have 
been  gathered,  a  clear  conception  of  the  causes 
has  been  formed,  from  which  may  be  drawn  defin- 
ite conclusions  as  to  the  means  of  prevention.  In 
a  number  of  the  cases  examined  serious  explo- 
sions have  occurred,  resulting  in  fire  and  death; 
in  others,  simply  ignition,  or  burning  out,  with  no 
other  damage  than  that  to  pipe  joints  and 
gaskets.  One  case  of  a  Bessemer  blowing  engine 
is  reported  where  the  pipes  have  been  red  hot  on 

3 


182012 


several  occasions  and  been  allowed  to  'burn  out. 
In  another  case  the  ignition  so  vitiated  the  air 
that  was  being  sent  into  a  mine  that  men  were 
asphyxiated.  The  report  upon  a  very  serious  ex- 
plosion states  that  "the  air  in  the  discharge  pipe 
first  caught  fire  and  then  exploded,"  it  having 
been  observed  before  the  explosion  that  the  pipe 
was  red  hot. 

It  would  be  out  of  place  in  this  discussion  to 
mention  the  location  of  these  accidents  or  the 
makes  of  the  compressors.  Such  reference  might 
be  construed  as  reflecting  on  the  maker  or  the 
management.  Furthermore,  some  of  the  informa- 
tion upon  which  this  article  is  based  was  given 
with  the  understanding  that  no  names  or  places 
should  be  mentioned. 

These  accidents  are  not  confined  to  any  one  type 
of  compressor,  nor  are  they  confined  to  cases 
where  the  terminal  gage  pressure  is  unusually 
high.  It  is  in  the  discharge  pipe  and  receiver  that 
the  trouble  occurs,  and  in  most  cases  the  com- 
pressor escapes  without  injury.  Rarely,  if  ever, 
does  the  explosion  take  place  immediately  in  the 
cylinder.  One  case  is  reported  where  the  cylinder 
head  was  blown  off,  killing  the  man  who  was  ex- 
amining the  discharge  valves,  but  even  in  this 
case  the  evidence  does  not  prove  that  the  explo- 
sion did  not  originate  in  the  receiver.  It  is  un- 
fortunate that  we  have  not  the  testimony  of  that 
man  as  to  the  condition  those  valves  were  in  at 
the  time,  for,  as  will  be  shown  later  on,  they  were 
probably  the  immediate  cause  of  the  explosion. 

It  is  perfectly  evident  that  just  before  such. an 
accident  there  must  be  something  to  burn,  in  the 
discharge  pipe  or  receiver,  and  a  temperature  suf- 
ficiently high  to  ignite  it.  Accumulation  of . com- 
bustible matter,  together  with  a  terminal  tem- 
perature at  the  end  of  a  compression  stroke  suf- 


ficient  to  start  combustion,  accounts  for  the  cases 
of  ignition;  the  presence  of  a  gas  or  vapor,  form- 
ing an  explosive  mixture  with  the  air,  with  a 
temperature  equal  to  the  burning  point  of  the. 
vapor  at  the  pressure,  accounts  for  the  cases  of 
explosion.  Where,  as  is  reported  in  a  few  cases, 
ignition  has  preceded  explosion,  it  is  clear  that  in 
the  first  place  there  was  a  sufficient  temperature 
to  start  combustion,  and  then  there  was  driven 
off  a  gas  or  vapor,  and  this,  mingling  with  the  air 
in  suitable  proportions,  formed  an  explosive  mix- 
ture. We  have,  then,  to  inquire  as  to.  the  origin 
and  nature  of  the  combustible  matter  and  to  the 
causes  and  conditions  that  tend  to  produce  ex- 
cessively high  temperature. 

ORIGIN   AND    NATURE    OF    THE    COMBUSTI- 
BLE MATTER. 

Some  lubrication  of  the  air  cylinder  is  neces- 
sary, and,  as  was  to  be  expected,  the  combustible 
matter  is  found  upon  examination  to  be  derived 
from  the  lubricant  used  together  with  dust  from 
the  air.  It  is  around  collieries  that  a  majority  of 
these  accidents  have  occurred,  but  enough  have 
occurred  where  the  air  was  practically  free  from 
dust  to  make  it  clear  that  while  dust,  and  espe- 
cially coal  dust,  undoubtedly  adds  an  element  of 
danger,  it  is  not  the  prime  source.  In  the  charac- 
ter and  quantity  of  the  lubricant  used  must  be 
located  the  first  cause  of  the  trouble.  The  dust 
aggravates  a  condition  first  produced  by  the  lubri- 
cant, and  the  lubricant  assists  the  dust  to  deposit. 
One  report  from  a  colliery  mentions  that  the  walls 
of  the  receiver  were  coated  arr  inch  thick  with  a 
caked  mass  of  dust  and  oil,  which  caught  fire  and 
burned  out  without  producing  an  explosion.  In 
another  case,  after  the  discharge  pipe  had  been 
wrecked  by  an  explosion,  the  deposit  on  the  re- 


ceiver  walls  was  found  to  be  2  ins.  thick.  These 
deposits  were  no  doubt  the  accumulation  of  years 
and  they  gave  no  trouble  until,  for  some  reason 
or  other,  there  was  a  rise  of  terminal  temperature. 
Then  there  was  "trouble  to  burn." 

It  will  not  be  argued  that  all  the  lubricant  used 
in  the  air  cylinder  is  deposited  on  the  walls  of  the 
receiver,  but  some  of  it  will  be,  and  the  more  lu- 
bricant used  the  more  will  be  deposited;  and  the 
greater  the  deposit  of  lubricant  the  greater  chance 
for  dust  to  adhere  and  produce  in  time  the  caked 
masses  above  referred  to.  Excess  of  lubrication, 
in  addition  to  waste,  means  increased  accumula- 
tion of  combustible  matter,  increased  lodgment 
of  dust  and  increased  danger  of  accident.  It  fol- 
lows that  just  as  little  lubricant  should  be  used 
as  possible,  and  inasmuch  as  an  air  cylinder  re- 
quires less  oil  than  a  cylinder  of  equal  size  using 
steam,  the  practice  on  the  steam  end  should  not  be 
followed  on  the  air  end. 

Two  lubricants  are  in  general  use  on  air  com- 
pressor cylinders,  a  mixture  of  soap  and  water 
and  cylinder  oil.  Either  can  be  made  to  do  the 
work.  The  mixture  of  soap  and  water  has  In- 
ferior lubricating  properties  and  must  be  used  in 
quantities  greatly  in  excess  of  those  required  of  a 
proper  lubricating  oil.  One  case  of  explosion  is 
reported  where  soap  and  water  was  used  almost 
exclusively.  A  test  of  the  deposit  found  in  the 
receiver  after  the  explosion  showed  that  it  read- 
ily ignited  at  400°  P.  This  deposit  was  2  ins. 
thick.  The  mixture  of  soap  and  water  was  also 
tested  in  a  laboratory.  Upon  being  evaporated 
down  to  a  "black,  semi-solid  mass"  it  ignited  at 
500°  F.  In  addition  to  this  soft-soap  lubricant 
there  had  been  used  some  oil  having  a  burning 
point  of  about  400°  F.  The  principal  combustible 
ingredient  in  the  deposit  was  coal  dust.  This  case 

6 


Is  sufficient  to  show  that  the  use  of  soap  and 
water  is  not  a  sure  preventative  of  accident.  In 
fact,  it  goes  further  and  shows  that  soap  and 
water  may  not  'be  as  good  as  oil,  for  the  tests 
showed  that  it  burned  readily  when  dry,  and 
inasmuch  as  more  of  it  would  have  to  be  used 
than  of  oil,  the  deposit  of  dust  would  be  corre- 
spondingly greater.  Nevertheless,  soap  and  water 
may  be  used  to  good  advantage  and  provision 
should  be  made  for  its  introduction  into  the  cyl- 
inder when  necessary. 

While  ignitions  are  bad  enough  and  to  be 
avoided  if  possible,  explosions  are  more  serious 
matters.  Evidently  they  are  due  to  the  formation 
of  a  mixture  of  a  combustible  gas  or  vapor  in 
suitable  proportions  with  air,  at  a  temperature 
sufficient  to  ignite.  The  percentage  of  gas  or 
vapor  required  to  make  an  explosive  mixture  is 
surprisingly  small  and  in  general,  the  heavier 
the  gas  or  vapor,  the  smaller  the  percentage. 
This  is  well  shown  in  the  following  table,  the  de- 
terminations being  made  by  Dr.  P.  Eitner,  of 
Carlsruhe: 

Table  Showing  the  Low  Limit  of  Explosibility  of  Certain 
Gases  and  Vapors  at  Normal  Temperature  and  At- 
mospheric Pressure. 

Per  cent,  by  volume. 

Hydrogen 8.5 

Marsh    Gas    6.3 

Ethylene 3.4 

Benzene  vapor  1.4 

Gasolene   vapor    

Benzol   vapor    1.1 

The  percentage  of  combustible  gas  or  vapor  re- 
quired to  produce  an  explosive  mixture  decreases 
with  the  temperature  of  the  mixture.  For  in- 
stance, at  ordinary  temperatures  16%  of  carbonic 
oxide  marks  the  low  limit  of  explosibility;  14.2% 
at  750°  F.;  9.3%  at  915°  F.;  7.4%  at  1,110°  F. 
Furthermore,  a  mixture  non-explosive  at  low 


pressure  becomes  explosive  at  higher  pressure. 
If,  then,  there  be  a  source  from  which  a  combus- 
tible gas  or  vapor  may  be  derived,  the  conditions 
of  pressure  and  temperature  which  obtain  in  the 
air  receiver  are  favorable  to  the  formation  of  an 
explosive  mixture  with  the  air.  And  as  shown 
above,  the  amount  required  is  very  small  indeed. 

Now,  combustible  gases  or  vapors  are  evolved 
from  all  lubricating  oils  by  heat.  The  lowest 
temperature  at  which  they  begin  to  come  off  is 
called  the  "flash  point"  of  that  oil.  At  a  some- 
what higher  temperature  the  gas  or  vapor  will 
ignite;  this  is  called  the  "burning  point."  The 
flash  point  of  kerosene  is  usually  below  150°  F. 
Ordinary  lubricating  oils  flash  at  about  250°  F. 
An  average  of  determinations  on  40  samples  of 
heavy  oils  having  average  flash  point  of  360°  F. 
gave  average  burning  point  of  398°  F. ;  high  flash 
test  cylinder  oils,  from  500°  F.  to  560°  F.,  gave 
burning  points  of  600°  to  630°  F.  The  vapors 
evolved  from  any  of  these  oils,  at  the  flash  point, 
or  the  oils  themselves,  atomized  or  sprayed,  would 
form  an  explosive  mixture  with  the  suitable  per- 
centage of  air.  From  the  above  table  it  is  evident 
that  as  low  as  1%  by  volume  would  form  such  a 
mixture.  If  the  oil  were  mixed  with  coal  dust 
the  temperature  at  which  such  a  deposit  would 
evolve  a  vapor  would  be  the  flash  point  of  the 
oil.  It  follows  that  none  but  high  flash  test  oil 
should  be  used  in  the  cylinder^ 

Makers  of  cylinder  oils  claim  that  some  oils 
have  a  greater  tendency  to  deposit,  or  to  "car- 
bonize," as  they  express  it,  than  others,  and  that 
the  flash  point  is  not  necessarily  a  gage  or  meas- 
ure of  this  "carbonizing"  tendency.  'Lubricating 
oils  are  extremely  complex  chemical  combinations 
of  carbon  and  hydrogen,  and  it  is  not  at  all  im- 
probable that  oils  having  the  same  flash  test 

8  M     .,  _; 


would  deposit  in  different  degree  upon  leaving  the 
cylinder.  Upon  this  point,  however,  no  exact 
data,  derived  from  experiment,  have  been  ob- 
tained. The  user  of  the  compressor  must  make 
his  own  observations  or  take  the  oil  manufac- 
turer's word  for  it. 

It  is  by  no  means  uncommon  for  the  engineer  in 
charge  of  an  air  compressor  to  introduce  through 
the  inlet  pipe  a  quantity  of  light  oil,  or  even  kero- 
sene. This  is  a  favorite  and  sometimes  effective 
way  of  cleaning  dirty  discharge  valves.  Under 
such  circumstances  it  is  possible  for  an  explo- 
sion to  take  place  immediately  in  the  cylinder,  for 
the  oil,  being  drawn  in  in  the  form  of  a  fine  spray 
or  mist,  and  compressed  with  the  air,  produces 
just  the  conditions  sought  after  in  the  cylinder  of 
an  oil  engine.  If  an  explosion  did  not  occur  it 
would  be  because  the  charge  "missed  fire,"  the 
terminal  temperature  of  compression  being  below 
the  ignition  point.  That  the  temperature  of  com- 
pression may  rise  to  the  ignition  point,  and  that, 
too,  in  very  few  strokes  of  the  compressor,  will 
be  shown  later.  If  an  explosion  did  not  occur  the 
light  oil  or  kerosene,  having  a  low  flash  point, 
being  added  to  the  accumulation  of  oil  and  dust 
already  in  the  receiver,  would  serve  to  lower  the 
temperature  at  which  some  time  in  the  future  a 
combustible  vapor  might  be  evolved.  Such  prac- 
tice is  simply  tempting  Providence;  and  Provi- 
dence, at  times,  refuses  to  resist  temptation. 
Among  the  cases  examined,  however,  there  is 
not  one  where  the  injection  of  light  oil  is  given 
as  the  immediate  cause  of  the  accident.  It  is 
hardly  to  be  expected  that  the  engineer  would  re- 
port that  his  ignorance  or  stupidity  caused  the 
trouble.  But  the  use  of  such  oils  for  such  pur- 
pose can  be  fairly  characterized  as- ignorance  and 
stupidity.  Considering  how  rerious  may  be  the 


results,  stronger  language  would  be  warranted. 
If  It  is  necessary  to  give  the  cylinder  a  heavy  dose 
of  lubricant  on  account  of  dirty  valves,  when  the 
compressor  cannot  be  shut  down  for  cleaning, 
soap  and  water  should  be  used,  forced  in  by  an 
oil  pump,  with  which  the  cylinder  should  be 
equipped,  in  addition  to  the  regular  lubricator. 

Special  care  should  be  given  to  the  design  of 
discharge  valves  and  clearance  spaces,  that  there 
may  be  no  small  pockets  or  recesses  where  oil 
may  accumulate.  This  also  applies  to  the  piping 
from  cylinder  to  receiver.  Bends  should  be 
avoided  as  far  as  possible,  for  a  change  in  direc- 
tion of  flow  increases  the  tendency  to  deposit. 

Despite  all  the  care  that  may  be  taken  as  re- 
gards lubrication,  there  remains  around  collieries 
the  dangers  due  to  the  accumulation  of  coal  dust. 
It  is  well  known  that  coal  dust  and  air  can  make 
an  explosive  mixture,  but  it  is  hardly  to  be  sup- 
posed that  the  air  in  the  receiver  could  be  suf- 
ficiently charged  with  dust  to  make  trouble.  Both 
anthracite  and  bituminous  coals  hold  varying 
quantities  of  combustible  gas,  mostly  marsh  gas 
or  "fire  damp,"  in  the  occluded  state,  that  is,  as 
free  gas,  not  chemically  combined.  'No  high  tem- 
perature, such  as  is  employed  in  destructive  dis- 
tillation in  the  manufacture  of  coal  gas,  is  re- 
quired to  liberate  this  occluded  gas.  In  a  vacuum 
it  may  be  driven  off  at  the  temperature  of  boiling 
water.  At  300°  F.  it  is  readily  evolved,  with  in- 
creasing evolution  at  higher  temperatures.  Such 
gas  production  would  be  entirely  harmless  unless 
a  temperature  of  ignition  were  reached.  But  coal 
dust  itself  ignites  at  about  500°  F.  At  this  tem- 
perature, ignition  followed  by  explosion  is  to  be 
expected.  The  evidence  in  two  cases  of  explosion 
indicates  that  this  is  exactly  what  occurred.  The 
blame  cannot  be  wholly  placed  upon  the  lubricant. 

10 


It  is  responsible  only  in  so  far  as  it  enabled  the 
dust  to  accumulate. 

Where  a  compressor  is  located  near  a  coal 
breaker  and  the  air  is  of  necessity  heavily  dust 
laden,  it  would  be  wise  to  consider  the  installation 
of  an  air  washer.  The  various  methods  of  air 
washing  cannot  be  discussed  here.  Suffice  it  to 
say  that  it  need  not  be  an  expensive  apparatus  to 
operate  or  maintain,  and  that  in  summer  time, 
with  a  suitable  supply  of  relatively  cool  water, 
the  reduction  in  temperature  of  the  inlet  air,  with 
consequent  increased  capacity  of  the  compressor 
and  corresponding  saving  of  power  for  a  given 
weight  of  air  compressed,  would  make  the  air 
washer  an  economical  device,  in  addition  to 
greatly  reducing  the  chance  of  accident. 

From  the  foregoing  it  would  appear  that  under 
the  best  of  conditions  there  is  always  the  possi- 
bility of  some  combustible  material  accumulating 
in  the  discharge  pipe  and  receiver,  and  under  the 
worst  conditions  the  certainty  that  the  deposit 
will  be  excessive.  Unless  ignited  it  is  harmless. 
What  are  the  causes  that  lead  to  ignition? 

TEMPERATURE    IN    DISCHARGE    PIPE    AND 
RECEIVER. 

The  temperature  due  to  compression  depends 
upon  three  factors:  Initial  temperature,  before 
compression;  pressure  to  which  the  air  is  com- 
pressed; efficiency  of  cooling  devices.  No  account 
will  be  taken  of  the  effect  of  moisture  in  the  air, 
and  all  temperatures  given  are  for  dry  air. 

The  initial  temperature  of  a  fresh  charge  of  air 
within  the  cylinder  is  always  above  that  of  the 
supply  from  which  it  is  drawn.  It  is  to  be  re- 
gretted that  no  accurate  means  of  determining 
the  temperature  immedately  before  compression 
has  been  devised.  The  inlet  valves  and  cylinder 

11 


walls  are  hot  from  the  previous  compression 
stroke,  and  in  coming  in  contact  with  these  heated 
surfaces  the  fresh  charge  is  heated.  How  serious 
may  be  the  effect  of  the  rise  in  initial  temperature 
due  to  this  cause  was  pointed  out  by  Mr.  Julian 
Kennedy,  in  a  paper  on  "Blowing  Engines,"  pre- 
sented before  the  World's  Engineering  Congress 
at  Chicago.  He  said: 

This  heating  of  the  incoming  air  expands  it  and  pro- 
portionately reduces  the  weight  of  air  entering  the  cylin- 
der at  each  stroke.  I  have  observed  this  in  the  case  of  an 
engine  which  was  so  constructed  as  to  cause  the  air  to 
travel  about  3  ins.  over  the  hot  metal  in  thin  films  3-16-in. 
thick.  Alongside  of  it  was  another  engine  of  the  same 
size  and  make  except  that  valves  were  used  that  allowed 
thft  air  to  pass  over  about  1  in.  of  metal,  the  openings 
being  of  such  size  that  each  stream  of  air  was  2  ins. 
in  thickness.  Careful  and  repeated  tests  of  these  engines, 
when  both  were  in  good  order,  showed  that,  while  the 
indicator-  diagrams  were  practically  the  same,  the  one 
with  the  large  valves  would  burn  about  10%  more  coke 
in  the  furnaces,  a  result  that  could  only  be  explained  on 
the  supposition  that,  in  the  case  of  the  engine  with  the 
small  air  openings,  the  incoming  air,  in  passing  through 
the  small  and  tortuous  passages  in  the  heads,  was  heated 
about  25°C.  more  than  in  the  case  of  the  other  engine. 

After  the  above  testimony,  further  comment  on 
the  rise  of  initial  temperature  due  to  contact  with 
hot  metal  surfaces  is  unnecessary. 

Some  rise  of  initial  temperature  is  frequently 
attributed  to  the  air  in  the  clearance  space,  inas- 
much as  this  air  is  at  the  temperature  of  dis- 
charge and  becomes  part  of  the  next  cylinder  full 
to  be  compressed.  When  air  expands  without 
doing  work,  the  slight  drop  in  temperature,  known 
as  the  Joule-Thomson  effect,  is  insignificant.  If, 
however,  it  expands  against  resistance,  doing 
work,  its  temperature  falls,  and  the  fall  is  propor- 
tional to  the  work  done.  With  mechanically- 
operated  inlet  valves,  set  to  open  the  instant  the 
return  stroke  started,  the  clearance  air  would  ex- 
pand wihout  doing  work  and  would  serve  to  raise 
the  initial  temperature  of  the  incoming  charge. 

12 


But  this  would  be  inexcusably  bad  valve  setting. 
With  poppet  valves,  opened  by  a  slight  vacuum 
within  the  cylinder,  the  clearance  air  expands 
against  the  piston  and  does  work,  and  the  ex- 
pansion being  very  rapid  it  is  presumably  adi- 
abatic.  Consequently,  there  is  a  fall  of  tempera- 
ture. In  fact,  owing  to  the  absorption  of  some 
heat  while  within  the  clearance  space,  it  is  pos- 
sible that  the  .clearance  air,  expanding,  reaches 
a  lower  temperature  than  that  of  the  charge  from 
which  it  was  derived.  In  well-designed  compres- 
sors clearance  space  is  reduced  to  a  minimum, 
for  the  sake  of  capacity,  so  that,  even  if  there 
were  no  drop  in  temperature  of  clearance  air,  the 
rise  of  initial  temperature  due  to  this  cause  can- 
not be  serious. 

The  place  from  which  the  air  is  drawn  may 
have  a  very  important  bearing  on  initial  tempera- 
ture. The  engine  room  is,  to  be  sure,  better  than 
the  boiler  room,  as  the  source  of  supply,  but  that 
is  about  all  that  can  be  said  in  favor  of  it.  The 
inlet  should  be  at  the  coolest  and  cleanest  place 
available.  A  difference  of  50°  between  the  engine 
room  and  the  outside  air  means  more  than  a  dif- 
ference of  50°  in  terminal  temperature,  as  well  as 
a  loss  of  about  10%  in  capacity  for  the  same 
amount  of  power  expended.  The  report  on  one 
explosion  makes  the  recommendation  that  the  air 
be  drawn  from  an  air  shaft,  cooled  with  water 
sprays.  Such  an  arrangement  might  be  adapted 
to  wash  the  air  at  the  same  time. 

The  effect  of  leaking  discharge  valves  upon 
initial  temperature  and  consequently  upon  the 
temperature  after  compression,  may  'be  very 
serious  indeed.  Suppose  an  extreme  case,  where 
the  amount  of  leakage  is  just  sufficient  to  main- 
tain atmospheric  pressure  within  the  cylinder  so 
that  no  fresh  air  enters.  The  initial  temperature 

13 


is  now  nearly  the  same  as  the  terminal  tempera- 
ture of  the  previous  charge,  for  the  compressed 
air,  in  leaking  'back,  has  done  no  work  upon  the 
piston,  and  consequently  has  not  dropped  any  in 
temperature.  The  hot  air  now  receives  a  second 
compression,  and  the  terminal  temperature, 
reached  by  starting  from  an  initial  temperature 
due  to  the  previous  stroke,  may  easily  reach  the 
point  of  ignition  of  the  combustible  matter.  If 
the  initial  temperature  were  60°  F.  and  terminal 
pressure  40  Ibs.,  the  terminal  temperature,  with 
no  cooling,  would  be  300°  F.  If  this  air  at  300°  F. 
leaks  back  and  compression  starts  from  that  tem- 
perature, the  temperature  of  discharge  becomes 
650°  F.  With  a  discharge  valve  stuck  open  it  Is 
plain  that  in  one  stroke  of  the  compressor  a  tem- 
perature might  be  reached  sufficient  to  ignite  the 
best  grade  of  high  flash  cylinder  oil. 

It  would  be  an  exceptional  case,  however,  in 
which  the  discharge  valves  leaked  so  badly  that 
there  was  no  introduction  of  fresh  air  at  all.  Yet 
to  the  extent  that  they  do  leak  is  the  initial  tem- 
perature raised.  Furthermore,  the  effect  is  cumu- 
lative, for  each  rise  in  initial  temperature  pro- 
duces a  greater  rise  in  terminal  temperature, 
which,  leaking  back  still  further,  raises  initial 
temperature,  and  so  on,  a  sort  of  endless  chain 
arrangement.  In  several  cases  of  accident  the 
discharge  valves  are  reported  as  having  been 
leaking,  and  in  one  case  an  examination  was 
being  made  at  the  time  the  explosion  occurred. 
The  results  of  the  examination  are  not  known, 
for  the  man  was  killed.  To  leaking  discharge 
valves,  more  than  to  any  other  cause,  may  be  at- 
tributed the  sudden  rise  of  temperature  in  the 
receiver,  which,  with  the  combustible  deposit 
there,  produces  an  accident. 

It  is  well  to  note  that  a  given  rise  in  initial  tem- 

14 


Ini-fial    Temperature  ,  Degrees  .  Fahrenheit. 


perature  produces  a  greater  rise  in  terminal  tem- 
perature. The  accompanying  curves  show  the 
rise  of  temperature  of  dry  air,  adiabatically  com- 
pressed, from  various  initial  temperatures  to  all 
gage  pressures  up  to  150  Ibs.  It  will  be  observed 
that  a  difference  of  40°  initial  temperature,  from 
60°  F.  to  100°  F.,  produces  a  difference  of  72°  ter- 
minal temperature,  at  100  Ibs.  pressure.  This 
tends  further  to  increase  the  cumulative  effect 
above  referred  to  as  due  to  leaking  discharge 
valves,  for  a  rise  of  1°  at  the  beginning  of  a 
stroke  involves  a  rise  of  nearly  2°  at  the  end  when 
compressing  to  100  Ibs.,  single  stage. 

It  will  be  observed  that  with  single  stage  com- 
pression from  60°  F.  initial  to  80  Ibs.,  without 
cooling,  a  temperature  of  430°  F.  as  attained.  This 
is  a  lower  initial  temperature  than  generally  ob- 
tained, yet  from  what  has  gone  before  it  is  clear 
that  such  a  terminal  temperature  is  beyond  the 
danger  line,  even  when  valves  are  tight. 

The  curves  also  show  the  rise  of  temperature 
with  pressure,  and  make  it  obvious  that,  other 
things  being  equal,  the  higher  the  pressure  the 
greater  the  likelihood  of  dangerously  high  tem- 
peratures in  the  receiver.  As  a  question  of  fact, 
there  is  greater  liability  to  accident  at  low  pres- 
sures than  at  high.  If  a  plant  were  to  be  in- 
stalled to  deliver  air  at  1,000  Ibs.  pressure,  stage 
compression  would  be  used,  the  questions  of  inter 
and  after-cooling  very  carefully  considered,  and 
all  precautions  taken  in  design  and  operation  to 
avoid  high  temperatures.  But  in  the  design,  in- 
stallation and  operation  of  a  single  stage  com- 
pressor, working,  say,  to  40  Ibs.,  such  precautions 
are  not  taken.  The  plant  does  not  command  re- 
spect. Yet,  as  shown  above,  in  a  few  strokes, 
with  a  sticking  discharge  valve,  a  temperature 
may  be  attained  sufficient  to  cause  everybody 

15 


within  reach  to  have  a  wholesome  respect  for  such 
a  magazine  of  explosive  energy  as  a  receiver  may 
become. 

From  the  foregoing  it  would  appear  that  all  pre- 
cautions having  been  taken,  we  must  look  for 
safety  from  accident  to  the  cooling  devices  em- 
ployed to  keep  down  terminal  temperatures.  De- 
spite all  the  care  that  can  be  taken,  some  com- 
bustible material  is  sure  to  accumulate  in  the  re- 
ceiver. Give  it  time  enough  and  it  will  get  there. 
It  cannot  be  expected  that  discharge  valves  will 
forever  and  always  remain  tight.  Reliance  must 
be  placed  upon  water  cooling. 

The  former  practice  of  injecting  water  into  the 
air  cylinder  has  been  abandoned  on  modern  ma- 
chines for  mechanical  reasons.  Water  jacketing 
of  heads  and  cylinders  on  large  machines  is  custo- 
mary, but  such  jacketing  is  entirely  insufficient 
to  insure  proper  cooling.  The  area  of  the  cool 
surfaces  is  so  small  relative  to  the  volume  of  air 
compressed  and  the  time  of  contact  is  so  short, 
lhat  the  cooling  effect  is  small.  But  water  jacket- 
ing is  most  efficient  in  keeping  down  the  tempera- 
ture of  the  metal  in  contact  with  the  fresh  charge, 
thus  keeping  down  initial  temperature.  This,  and 
the  aid  rendered  lubrication  by  keeping  cylinder 
walls  comparatively  cool,  are  the  chief  services 
performed  'by  the  water  jackets.  On  large  ma- 
chines they  cannot  be  depended  upon  to  absorb 
the  heat  of  compression.  There  is  too  much  heat 
for  the  time  and  surface. 

Compression  by  stages,  with  inter-cooling,  with 
provision  for  after-cooling  in  an  emergency,  are 
the  surest  means  by  which  to  avoid  dangerously 
high  temperatures  in  the  receiver.  The  curves 
show  the  reduction  in  terminal  temperature  that 
may  be  attained  by  inter-cooling  to  any  tempera- 
ture between  stages,  and  inasmuch  as  for  adia- 

16 


batic  compression,  the  rise  of  temperature  is  pro- 
portional to  work  expended,  they  also  show  to 
what  pressure  each  cylinder  should  operate  in 
order  that  each  shall  do  an  equal  amount  of  work. 
Knowing  the  weight  of  air  compressed  per  stroke 
and  its  specific  heat,  the  curves  also  give  the  nec- 
essary data  from  which  to  calculate  the  British 
thermal  units  to  be  absorbed  by  the  inter-cooler. 

The  economies  that  may  be  effected  by  stage 
compression  are  well  known,  but  it  must  be  re- 
membered that  the  economy  depends  upon  the 
efficiency  of  the  inter-cooler.  Without  inter- 
cooling,  stage  compression  effects  no  economy 
over  single  stage.  That  the  inter-cooler  may  be 
efficient,  it  must  have  ample  cooling  surface,  and 
it  may  be  remarked  here  that  for  the  sake  of  first 
cost,  to  cut  down  the  size  of  the  inter-cooler,  as 
is  the  practice  of  some  manufacturers,  is  the  poor- 
est kind  of  economy.  As  well  expect  to  get  boiler 
economy  by  cutting  down  heating  surface.  Econ- 
omy in  both  cases  depends  upon  the  transference 
of  heat  to  water,  through  metal.  The  amount 
transferred,  other  things  being  equal,  depends 
upon  the  surfaces  in  contact. 

The  after-cooler  may  be  similar  in  construction 
to  the  inter-cooler,  or,  if  water  at  sufficient  pres- 
sure is  available,  it  may  be  a  simple  water  spray 
so  arranged,  like  a  sprinkler  head,  to  throw  a  fine 
mist  or  spray  into  the  discharge  pipe.  This  spray 
need  only  be  used  in  an  emergency  when  the  tem- 
perature in  the  discharge  pipe  has  risen  to  the 
danger  point.  A  steam  jet  will  answer  the  same 
purpose,  but  owing  to  the  latent  heat  of  evapora- 
tion, a  given  weight  of  water  will  have  greater 
heat  absorptive  power  than  the  same  weight  of 
steam.  Ample  provision  must  be  made  upon  the 
receiver  to  drain  off  this  cooling  water,  but  the  re- 
ceiver should  have  an  ample  drain  anyhow,  and  be 

17 


large  enough  and  so  designed  as  to  act  as  a  sepa- 
rator. Only  in  very  exceptional  cases  would 
the  addition  of  a  cooling  spray  to  the  dis- 
charge pipe  increase  the  moisture  content  in 
the  air  from  the  receiver.  Usually  the  air 
is  already  at  the  point  of  saturation.  It 
cannot  be  any  wetter  than  it  is.  This  is 
proven  by  the  fact  that  water  generally  accumu- 
lates in  the  receiver.  The  air  is  already  at  the 
"dew  point"  and  cannot  hold  any  more  water  at 
the  temperature  and  pressure.  In  fact,  it  is  en- 
tirely possible  to  cool  the  air  in  the  receiver  by 
the  addition  of  water,  so  that  the  moisture  con- 
tent will  be  reduced  and  the  air  at  the  drills  or 
mining  machines  will  be  actually  dryer  than  if  no 
water  had  been  added.  With  a  receiver  built  upon 
the  lines  of  a  steam  separator,  this  result  could 
be  readily  accomplished,  the  dangers  from  explo- 
sion practically  eliminated,  and  the  troubles  due 
to  "freezing  up"  reduced  to  a  minimum. 

Thus  we  reach  the  somewhat  curious  conclusion 
that  the  troubles  due  to  heat,  namely,  ignitions, 
and  the  troubles  due  to  cold,  namely,  "freezing 
up,"  may  both  be  greatly  alleviated  by  the  same 
remedy,  namely,  adequate  after-cooling. 

If  the  after-cooling  is  to  be  done  by  a  spray 
and  only  used  when  required,  it  is  necessary  to  in- 
stall thermometers  on  discharge  pipes,  that  the 
temperatures  may  be  observed.  Recording  ther- 
mometers would  be  preferable  for  such  purposes, 
but  mercury  thermometers,  with  large  gradua- 
tions, reading  up  to  1,000°  F.,  are  not  expensive, 
and  would  serve  the  purpose.  They  should  be 
capable  of  being  read  from  a  distance,  and  the 
engineer  should  watch  them  as  he  would  a  steam 
gage. 

That  the  liability  to  ignition  and  explosion  may 
be  reduced  to  a  minimum,  the  foregoing  consider- 

18 


ations  warrant  the  following  conclusions  relative 
to  the  design,  installation  and  operation  of  air 
compressors: 

DESIGN  OF  COMPRESSORS.— (1)  Clearance 
space  should  be  reduced  to  a  minimum.  (2)  In- 
going air  should  traverse  as  small  a  surface  of 
hot  metal  as  possible.  (3)  Discharge  valves  and 
passageways  should  contain  no  pockets  or  recesses 
for  the  accumulation  of  oil.  (4)  Cylinders  and 
heads  shoud  be  water  jacketed;  in  some  cases 
piston  water  cooling  may  be  resorted  to.  (5)  Stage 
compression,  with  adequate  inter-cooling,  should 
be  employed  wherever  final  pressure  and  first  cost 
of  installation  will  warrant.  (6)  Discharge  valves 
must  be  easy  of  access  for  cleaning  and  examina- 
tion. There  must  be  no  excuse  for  dirty  or  leaky 
valves. 

INSTALLATION  OF  COMPRESSOR.— (1)  Air 
should  be  drawn  from  the  coolest  and  cleanest 
place  possible,  and  never  from  the  engine  room. 
Engine  room  air  is  never  cool  nor  clean,  and  an 
open  intake  is  a  constant  invitation  to  squirt  oil 
in  from  a  can.  (2)  Around  collieries  it  would  be 
well  to  consider  the  washing  of  the  air.  (3)  A 
thermometer,  preferably  recording,  should  be 
placed  on  the  discharge  pipe.  (4)  Provision  for 
after-cooling  should  be  made;  a  water  spray  will 
answer,  to  'be  used  when  the  thermometer  indi- 
cates the  necessity.  (5)  The  receiver  should  be 
provided  with  a  man  head  for  cleaning,  and  a 
drain  easy  of  access  and  ample  in  size.  (6)  Auto- 
matic sight  feed  lubricators  should  be  depended 
upon  for  regular  lubrication,  'but  in  addition  an 
oil  pump  may  be  installed  for  the  introduction 
of  soap  and  water  in  case  of  necessity. 

OPERATION  OF  THE  COMPRESSOR.— (1) 
High-flash  test  cylinder  oil  alone  should  be  used 
for  regular  lubrication.  Under  no  circumstances 
must  kerosene  or  light  oil  be  introduced.  If  an 

19 


extra  heavy  dose  of  lubricant  is  required,  give  It 
soap  and  water  through  the  oil  pump.  (2)  Dis- 
charge valves  must  be  kept  tight,  and  to  this  end 
the  use  of  the  steam  engine  indicator  is  advised. 
The  cards  may  not  tell  much  about  the  condition 
of  the  valves,  but  one  of  the  greatest  values  of 
the  indicator  is  the  moral  effect  upon  the  engi- 
neer. (3)  Discharge  valves  should  be  cleaned  from 
dust  and  oil  and  frequent  examinations  made  to 
see  if  they  need  it.  (4)  Accumulation  of  water  and 
oil  must  be  blown  from  the  receiver  and  an  inter- 
nal examination  made  at  stated  intervals.  (5) 
The  thermometer  should  be  watched  like  a  steam 
gage.  Before  it  reaches  400°  F.  get  busy.  This  is 
the  danger  limit.  Put  on  the  after-cooling  spray, 
examine  all  water  supply  pipes  and  the  discharge 
valves.  (6)  After  the  designer  has  done  all  in  his 
power  to  make  a  perfect  machine,  and  after  the 
purchaser  has  taken  all  reasonable  precautions 
to  install  it  with  every  safeguard  against  accident, 
the  responsibility  must  rest  on  the  engineer  in 
charge.  He  should  be  thoroughly  instructed  as  to 
the  possibility  of  explosion,  the  dangers  attendant 
upon  the  use  of  any  but  the  prescribed  oil,  and  the 
effect  of  leaking  discharge  valves.  He  should  be 
instructed  in  the  use  of  the  steam  engine  indicator 
and  required  to  submit  cards  at  stated  intervals. 
He  should  record  in  the  engine  room  log  the  daily 
condition  of  the  machines  under  his  charge.  He 
should  be  given  a  wholesome  respect  for  an  air 
compressor,  with  imperative  instructions  to  keep 
it  clean,  inside  as  well  as  out. 

It  may  be  objected  that  some  expense  is  in- 
volved in  following  all  the  above  recommenda- 
tions and  suggestions.  In  the  preparation  of  this 
paper  about  twenty  different  concerns  have  been 
found  that  have  learned  from  experience  that  ex- 
plosions and  ignitions  are  expensive.  Very  many 
more  could  give  the  same  testimony. 


AN  LIT1AL  FINE  i  01  ;  JI6CHT8 

FOR   FA.LURE 
DU  E 

SEVENTH 


W.U.   BE 

BOOK  ON   THE 


OVERDUE. 


LD  21-95m-7,'37 


18201 


